Hmm very interesting reading and I applaude your calculations.
However the trick is getting close enough to fire your grape, I would estimate and I do mean estimate the enemy ship could get in two braodsides before you get into that range and then would not just sit there taking grape without reprisal.
So yes I can understand maybe three frigates taking down a 1st or 3rd rate, I shall have to have a go and see the results.
Nicely done Hugh